COMPOSITION OF MILK

1. What composes the milk solids?

2. What composes the major part of the milk protein?

3. What is the difference between milk curd and whey?

4. What is the principal carbohydrate in milk?

5.

6. What vitamins are normally found in ruminant milk and are not associated with the diet?

7. How do off-flavors get into ruminant milk?

8. Why does milk from marine mammals have a high concentration of fat?

9. What is colostrum and what are its highlights?

The gross composition of milk refers to the proportions of water, fat, carbohydrate, protein, and minerals it contains. Water content is determined by the loss of weight observed when milk is dried. Fat content is determined by extraction with defined methods. Carbohydrate in milk is usually expressed as the lactose equivalent and can include other carbohydrates. Protein content represents all proteins, including enzymes. Milk minerals are usually expressed as ash, which represents the residue remaining after incineration. Tables that show the composition of milk often present only the percentages of fat, protein, lactose, and ash and delete water. The aggregate of fat, protein, lactose, and ash is referred to as dry matter, or milk solids.

Proteins

The caseins (alpha, beta, gamma, and kappa) constitute the major part of the milk proteins. These protein fractions are insoluble at a pH of 4.6 and make up what is known as the curd. The other proteins are alpha-lactalbumin, beta-lactoglobulin, blood serum albumin, immunoglobulins, and a proteose-peptone fraction. These other proteins are soluble at pH 4.6 and are referred to as the whey proteins. The immunoglobulins are present in very small amounts, except in colostrum. All of the proteins.are synthesized in the mammary gland from amino acids except gamma-casein, blood serum albumin, and immunoglobulins (the immunoglobulin fraction of colostrum, however, is synthesized in the mammary gland).

Carbohydrates

The principal carbohydrate in milk is lactose. It is synthesized in the mammary gland. Lactose is a disaccharide that contains glucose and galactose moieties. It is unique to the mammary gland, but small amounts are found in plasma during lactation. The principal precursor of lactose in the blood is glucose; propionate is also a precursor, by way of glucose. Propionate is important in ruminants because of its availability from fermentative processes in the rumen.

Lipids

Milk lipids consist primarily of triglycerides. Other lipids include small amounts of phospholipids, cholesterol, free fatty acids, monoglycerides, and fat-soluble vitamins. Milk fat synthesis in ruminants proceeds mostly from acetic and butyric acids. Acetic acid constitutes about 60%.to 70% of the volatile fatty acids from rumen fermentation. A reduction in milk fat concentration occurs when fermentation changes cause a decrease in production of acetic acid.

Minerals

The major minerals in cow’s milk are calcium (0.12%), phosphorus (0.10%), sodium (0.05%), potassium (0.15%), and chlorine (0.11%). Other minerals found in trace amounts include magnesium, sulfur, copper, cobalt, iron, iodine, and zinc.

Vitamins

The B vitamins and vitamin K are synthesized by ruminants and their concentration in milk is not influenced by diet. Vitamin K is also synthesized by the intestine, so its presence in nonruminant milk does not depend on diet. Vitamins A, D, and E are not synthesized in the rumen, so their presence in milk does depend on the diet. The amount of vitamin C in milk is not greatly influenced by diet.

Other Substances

Many drugs pass readily into the milk from the blood. Milk must therefore be withdrawn from the market for a certain period when cows have been treated with specific drugs, particularly antibiotics.

Off-flavors are sometimes detected in milk when cows have eaten certain foods.

Species Variation in Composition

The approximate gross composition of milk is presented in Table 16-1 for several domestic animals, whales (marine mammal), and humans. The milk of marine mammals has a high fat content (33.2% in the whale as compared with 3.5% in the Holstein cow), which is a consequence of the concentration necessary to conserve isotonic water. The isotonic water intake of marine mammals is restricted and is limited to that obtained from the fish they eat, so conservation is required. Lactose composition is probably the most constant among species but still varies considerably. Some have thought that milk with a high protein content is characteristic of species with fast-growing offspring, but this is not a consistent finding. Variations are apparent among breeds within a species (e.g., fat and protein when comparing Holstein and Guernsey cows). Differences exist among individuals within a breed and within an individual, depending on the stage of lactation and on whether the milk drawn from the udder is first drawn or last drawn. Last-drawn milk in cows has a higher fat percentage. Also, the fat composition in cow’s milk is higher during the first 2 weeks after parturition, decreases slightly for 3 to.4 months, and gradually decreases thereafter. Fat and protein content of milk is not affected by altering the fat and protein content in the diet.

Modified from Jacobson NL, McGilliard AD.

Colostrum

Colostrum has been variously defined, but it is considered to be the initial mammary secretion after parturition. The composition of colostrum is decidedly,different from milk composition that is considered normal for the species. The differences apparent in colostrum persist in descending magnitude for 4 to 6 days after parturition.

Colostrum is high in the whey proteins, particularly the immunoglobulins. Passive immunity is transferred to the offspring from the mother by immunoglobulins in colostrum. The period during which absorption occurs extends for 1 to 2 days after birth in the pig, horse, cow, and dog. Under normal circumstances, the period is estimated to be 4 days or less in sheep and goats. Beyond this time, the immunoglobulins are more subject to digestion by proteolytic enzymes. Other significant differences between colostrum and regular milk are its higher concentrations of vitamin A, vitamin E,,carotene, and riboflavin. Generally, colostrum also contains more protein, ash, and fat and less lactose than regular milk.

■